Heat-treating method for improving wear-resistance and corrosion-resistance of chromium-plated steel substrate

ABSTRACT

Disclosed is a method for improving the wear- and corrosion-resistance of a chromium-plated steel substrate by heat-treating the chromium-plated steel substrate under optimum conditions to inhibit a drop in corrosion-resistance of a steel substrate due to fine cracks formed at a chromium layer, and to improve a hardness of the chromium layer. The heat-treating method comprises the steps of: plating the chromium layer onto the steel substrate; and heating the chromium-plated steel substrate in an oxidizing gas environment at above atmospheric pressure to form oxidized layers containing magnetite (Fe 3 O 4 ) on the surface of the steel substrate, the surface of the steel substrate being partly exposed to the air through penetrating cracks formed in the chromium layer.  
     In accordance with the heat-treating method, the chromium-plated steel substrate having excellent corrosion- and wear-resistance can be easily obtained by plating chromium onto the steel substrate, followed by oxidizing the chromium-plated steel substrate. In addition, the heat-treating method contributes to fewer defects, low manufacturing costs and long life span of chromium plating-related products.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a heat-treating method forimproving the wear- and corrosion-resistance of a chromium-plated steelsubstrate. More particularly, the present invention relates to a methodfor improving the wear-and corrosion-resistance of a chromium-platedsteel substrate by heat-treating the chromium-plated steel substrateunder optimum conditions to inhibit a drop in corrosion-resistance of asteel substrate due to fine cracks formed in a chromium layer, and toimprove a hardness of the chromium layer as well.

[0003] In general, chromium platings, which have been widely used invarious industrial and decorative fields, are largely classified intotwo categories: a hexa-valent chromium plating process and a tri-valentchromium plating process. The hexa-valent chromium plating process hasbeen mainly applied since 1920s, while the tri-valent chromium platingprocess was recently developed to reduce environmental pollution.

[0004] In particular, since the chromium platings used for variousindustrial fields have excellent resistance to corrosion and wear, theyare currently used in moldings, automobiles and various machine parts.In addition, decorative chromium platings exhibit higher excellentresistance to corrosion than any other metal plating.

[0005] The hexa-valent chromium plating is advantageous in terms oflow-cost treatment and excellent resistance to corrosion, wear and heat.However, fine cracks are likely to occur in a hexa-valent chromium layerin view of properties intrinsic to the chromium plating. The fine crackslower a resistance of the steel substrate to corrosion, which results inthe steel substrate being rusted.

[0006] On the other hand, a tri-valent chromium layer exhibits ahardness similar to the hexa-valent chromium layer. Like the hexa-valentchromium layer, the tri-valent chromium layer has a low resistance tocorrosion due to the presence of fine cracks formed therein. Thehardness of the hexa-valent chromium layer is reduced by a relaxedresidual stress inside the layer at high temperature. However, thehardness of the tri-valent chromium layer is increased by a small amountof carbon present in an organic complexing agent of a plating bath. Thatis, the carbon is incorporated into the tri-valent chromium layer, andthe Cr—C platings thus formed are crystallized at high temperature toform highly hard chromium carbide precipitates such as Cr₇C₃, Cr₂₃C₆,etc. The precipitates increase the corrosion-resistance of thetri-valent chromium layer. At this time, increased hardness due toprecipitation of stable carbides improves wear-resistance of thetri-valent chromium layer.

[0007] Although the hardness and corrosion-resistance of the tri-valentchromium layer are increased as described above, corrosion-resistance ofa tri-valent chromium-plated steel substrate is decreased due to thepresence of fine cracks formed at the tri-valent chromium layer.

[0008] Therefore, the present invention has been made in view of theabove problems, and it is an object of the present invention to providea heat-treating method for improving the wear- or corrosion-resistanceof a chromium-plated steel substrate by plating hexa-valent ortri-valent chromium onto a steel substrate, followed by oxidizing thesurface of an steel substrate exposed to the air through fine cracksformed at a chromium layer to form Fe₃O₄ layers on the surface of thesteel substrate. In accordance with the heat-treating method, thecorrosion-resistance of hexa-valent chromium-plated steel substrate isimproved, and the corrosion- and wear-resistance of tri-valentchromium-plated steel substrate are improved.

[0009] To achieve the above objective, there is provided a heat-treatingmethod for improving the wear- and corrosion-resistance of achromium-plated steel substrate, comprising the steps of:

[0010] plating a chromium layer onto an steel substrate; and

[0011] heating the chromium-plated steel substrate in an oxidizing gasenvironment at above atmospheric pressure to form oxidized layerscontaining magnetite (Fe₃O₄) on the surface of the steel substrate, thesurface of the steel substrate being partly exposed to the air throughpenetrating cracks formed in the chromium layer.

[0012] In accordance with the heat-treating method according to thepresent invention, the step of heating in an oxidizing gas environmentto form oxidized layers is preferably carried out at 180˜570° C. for10˜600 minutes.

[0013] The oxidized layers formed by heating in an oxidizing gasenvironment preferably contain magnetite (Fe₃O₄) in an amount of above60% by weight.

[0014] The oxidizing gas used in the present invention is preferablysteam, carbon dioxide, air, or mixtures thereof.

[0015] In accordance with one aspect of the present invention, theheat-treating method for improving the wear- and corrosion-resistance ofthe chromium-plated steel substrate further comprises the step ofcarbonitriding the chromium-plated steel substrate prior to heating inan oxidizing gas environment to form oxidized layers. The carbonitridingstep permits formation of iron nitrides (Fe_(2˜3)N) in the form of aε-phase on the surface of the steel substrate exposed to the air throughfine cracks formed in the chromium layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

[0017]FIG. 1 is a cross-sectional view schematically showing achromium-plated steel substrate in which a penetrating crack is formed,prior to applying a heat-treating method according to the presentinvention;

[0018]FIG. 2 is a cross-sectional view schematically showing aprotective layer formed on the surface of an steel substrate by aheat-treating method for improving the wear- and corrosion-resistance ofa chromium-plated steel substrate according to the present invention;

[0019]FIG. 3 is a scanning electron microscopic image showing achromium-plated steel substrate in which penetrating cracks are formed,prior to applying a heat-treating method according to the presentinvention; and

[0020]FIG. 4 is a scanning electron microscopic image showing protectivelayers formed on the surface of an steel substrate by a heat-treatingmethod for improving the wear- and corrosion-resistance of achromium-plated steel substrate according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, the present invention will be explained in moredetail with reference to the accompanying drawings.

[0022]FIG. 1 is a cross-sectional view schematically showing achromium-plated steel substrate in which a penetrating crack is formed,prior to applying the heat-treating method according to the presentinvention; and FIG. 2 is a cross-sectional view schematically showing aprotective layer formed on the surface of an steel substrate by theheat-treating method for improving the wear- and corrosion-resistance ofa chromium-plated steel substrate according to the present invention.

[0023] Generally, a hexa-valent chromium layer is formed on the surfaceof a steel substrate as follows. First, the steel substrate as a cathodeand an insoluble anode are dipped in a plating bath consisting ofanhydrous chromium trioxide (CrO₃) and water (H₂O) Electricity is thenapplied between the cathode and anode to form chromate ion (CrO₄ ²⁻)through a chromic acid (H₂CrO₄) intermediate. The chromate ion (CrO₄ ²⁻)is deposited on the surface of the steel substrate in the form of Crmetal. In the case of a tri-valent chromium layer, an organic compoundsuch as formamide (HCONH₂) is further added as a carbon source to theplating bath.

[0024] As shown in FIG. 1, the chromium layer thus formed has finecracks therein. The steel substrate is subjected to corrosion in acorrosive environment due to the fine cracks.

[0025] However, as shown in FIG. 2, formation of iron oxide (Fe₃O₄) onthe surface of the steel substrate prevents the steel substrate fromcontacting with the corrosive environment. Iron oxide (Fe₃O₄) is acompound, whic is highly resistant to corrosion. Accordingly, thehexa-valent chromium-plated steel substrate has excellentcorrosion-resistance.

[0026] On the other hand, when the tri-valent chromium-plated steelsubstrate is oxidized at high temperature, crystallization occurs in thefine cracks formed at the chromium layer and carbon is bonded to Crmetal to form highly hard chromium carbides such as Cr₇C₃, Cr₂₃C₆, etc.Accordingly, the tri-valent chromium-plated steel substrate has improvedwear-resistance.

[0027] As described above, when the hexa- or tri-valent chromium-platedsteel substrate is heated in a furnace at 180˜570° C. ° C. for 10˜600minutes, iron oxides including Fe₃O₄ as a main component are formed onthe surface of the steel substrate which contacts with a corrosiveenvironment through the fine cracks formed at the chromium layer.

[0028] When the oxidation of the chromium-plated steel substrate iscarried out at a temperature lower than 180° C., the oxidation rate isso slow that the thickness of the oxidized layers formed isinsufficient, and further the hardness of the tri-valent chromium layeris unsatisfactory.

[0029] When the oxidation of the chromium-plated steel substrate iscarried out at a temperature higher than 570° C., wustite (FeO) havinglow corrosion-resistance is formed on the surface of the steelsubstrate.

[0030] Examples of the furnace usable in the present invention includefurnaces at 1˜100 atm, vacuum furnaces at below atmospheric pressure andplasma furnaces. All these furnaces include oxidizing gas such as air,oxygen and steam, and preferably include steam.

[0031] With regard to a time for heat-treating the chromium-plated steelsubstrate , the heat-treatment is carried out for a time sufficient forthe formation of the oxidized layer containing magnetite (Fe₃O₄) in anamount of above 60% by weight on the surface of the steel substrate. Forexample, at a heat-treatment temperature of 570° C., the chromium-platedsteel substrate is heated for 10 minutes or longer and preferably 30minutes to form desired oxidized layers. At 180° C., the chromium-platedsteel substrate is heated for at least 600 minutes.

[0032] In addition, carbonitriding the chromium-plated steel substrateprior to oxidizing it leads to the formation of iron nitrides(Fe_(2˜3)N) in the form of a ε-phase on the surface of the steelsubstrate exposed to a corrosive environment through fine cracks formedat the chromium layer. Accordingly, the carbonitrided chromium-platedsteel substrate is highly resistant to corrosion.

[0033] Carbonitridation of the chromium-plated steel substrate iscommonly carried out at 450˜650° C. for 30˜300 minutes. In particular, vwhen carbonitriding at above 570° C., the oxidation of thechromium-plated steel substrate is preferably carried out at atemperature lower than 570° C.

EXAMPLE

[0034] Sargent bath (CrO₃ 250 g/l , H₂SO₄ 2.5 g/l) was used as a platingbath for plating hexa-valent chromium on a steel substrate , and aplating bath containing 150 g/l CrO₃, 1.5 g/l H₂SO₄ and 10˜20 ml offormamide (HCONH₂) as a carbon source was used for plating tri-valentchromium on a steel substrate , respectively. Lead was used as anodes inboth baths. Specimens for both chromium plating were subjected toreverse electrolysis at a current density of 10A/dm² for 1 hour,followed by at a current density of 40˜60A/dm² and 45˜55A/dm² ° C. toobtain a hexa-valent and a tri-valent chromium-plated steel substrate ,respectively.

[0035]FIG. 3 is a scanning electron microscopic image showing thechromium-plated steel substrate in which penetrating cracks are formed,prior to applying the heat-treating method according to the presentinvention; and FIG. 4 is a scanning electron microscopic image showingprotective layers formed on the surface of the steel substrate by theheat-treating method for improving the wear- and corrosion-resistance ofthe chromium-plated steel substrate according to the present invention.

[0036]FIGS. 3 and 4 show cross-sectional views before and after theoxidation of the chromium-plated steel substrate, respectively. As shownin FIGS. 3 and 4, a number of fine cracks were observed in both thetri-valent and hexa-valent chromium layers.

[0037] Specimens of the tri-valent and hexa-valent chromium-plated steelsubstrate thus obtained were carbonitrided and then oxidized using steamunder the conditions shown in Table 1 below. For comparison, specimensof the chromium plated steel substrate were directly oxidized usingsteam under the conditions shown in Table 1 below, without anycarbonitriding step. TABLE 1 Hardness and corrosion tests of tri-valentand hexa- valent chromium-plated steel substrate at various conditionsfor heat-treatment Types of Hardness Salt chrom um Heat (Test load sprayplat ngs treatment cond t ons 300 g) test Remarks Tr -valent Unox d zedHV800 Below Before chrom um  6 hrs. treatment plat ng Ox d zed at 150°C. HV850  15 hrs. Comparat ve ° C. for 10 hrs. example Ox d zed at 180°C.  HV1000  60 hrs. Invent ve ° C. for 10 hrs. examples Ox d zed at 250°C.  HV1500 150 hrs. ° C. for 5 hrs. Ox d zed at 350° C.  HV1790 500 hrs.° C. for 3 hrs. Ox d zed at 450° C.  HV1850 1000 hrs.  ° C. for 2 hrs.Carbon tr ded at  HV1830 Above 570° C. ° C. for 3 1250 hrs.  hrs., andthen ox d zed 520° C. ° C. for 30 m n. Ox d zed at 570° C.  HV1830 Above° C. for 30 m n. 1030 hrs.  Ox d zed at 600° C.  HV1850 Below Comparatve ° C. for 30 m n.  10 hrs. example Hexa-valent Unox d zed HV750 BelowBefore chrom um  6 hrs. treatment plat ng Ox d zed at 150° C. HV700  10hrs. Comparat ve ° C. for 10 hrs. example Ox d zed at 180° C. HV680  50hrs. Invent ve ° C. for 10 hrs. examples Ox d zed at 350° C. HV657 450hrs. ° C. for 3 hrs. Ox d zed at 450° C. HV425 800 hrs. ° C. for 2 hrs.Carbon tr ded at HV380 Above 570° C. ° C. for 3 1250 hrs.  hrs., andthen ox d zed 520° C. ° C. for 30 m n. Ox d zed at 570° C. HV385 Above °C. for 30 m n. 1030 hrs.  Ox d zed at 600° C. HV380 Below Comparat ve °C. for 30 m n.  10 hrs. example

[0038] As a result, the tri-valent chromium-plated steel substrate wasobserved to exhibit very high hardness of above HV1800, and excellentcorrosion-resistance of 1030˜1050 hours in a salt spray test, whichdemonstrates 170 times higher than the unoxidized chromium-plated steelsubstrate.

[0039] When the oxidation of the chromium-plated steel substrate wascarried out at a temperature lower than 180° C., the oxidation rate wasso slow that the thickness of the oxidized layers formed wasinsufficient, and further the hardness of the tri-valent chromium layerwas unsatisfactory.

[0040] When the oxidation of the chromium-plated steel substrate wascarried out at a temperature higher than 570° C., the chromium-platedsteel substrate exhibited poor results in the salt spray test due toformation of wÜustite (FeO) having low corrosion-resistance on thesurface of the steel substrate.

[0041] Like tri-valent chromium-plated steel substrate, the hexa-valentchromium-plated steel substrate was observed to exhibit very highhardness of above HV380, and excellent corrosion-resistance of 1030˜1050hours in a salt spray test, which demonstrates 170 times higher thanconvenient hexa-valent chromium-plated steel substrate.

[0042] In particular, it was observed that the higher the oxidationtemperature of the hexa-valent chromium-plated steel substrate was, thelower was the hardness of the hexa-valent chromium-plated steelsubstrate. Accordingly, appropriated reduction in the oxidationtemperature of the hexa-valent chromium-plated steel substrate can leadto excellent corrosion-resistance without any influence on the hardnessof the hexa-valent chromium-plated steel substrate.

[0043] Protective oxidized layers were formed by carbonitriding andoxidizing the chromium-plated steel substrate under the conditions shownin Table 1. FIG. 4 shows the protective oxidized layers (marked byarrow) consisting of iron nitrides (Fe_(2˜3)N) in the form of a ε-phaseand magnetite (Fe₃O₄) on the surface of the steel substrate exposed tothe air through fine cracks formed at the chromium layer. Due toexcellent corrosion-resistance of the protective oxidized layers, thechromium-plated steel substrate exhibits excellent corrosion-resistance,too.

[0044] As described above, the heat-treating method enabled thehexa-valent chromium-plated steel substrate to exhibit acorrosion-resistance 170 times higher than the unoxidizedchromium-plated steel substrate. In particular, the tri-valentchromium-plated steel substrate exhibited very high hardness of aboveHV1800, and a corrosion-resistance 170 times higher than the unoxidizedchromium-plated steel substrate.

[0045] In accordance with the heat-treating method according to thepresent invention, the chromium-plated steel substrate having excellentcorrosion- and wear-resistance can be easily obtained by platingchromium onto the steel substrate, followed by oxidizing thechromium-plated steel substrate. Accordingly, additional steps includingremoving the chromium layer and re-plating chromium onto the steelsubstrate, which are problems of conventional chromium-plated steelsubstrate,

[0046] Although the preferred embodiments of the present invention havebeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A heat-treating method for improving the wear-and corrosion-resistance of a chromium-plated steel substrate,comprising the steps of: plating a chromium layer onto an steelsubstrate; and heating the chromium-plated steel substrate in anoxidizing gas environment at above atmospheric pressure to form oxidizedlayers containing magnetite (Fe₃O₄) on the surface of the steelsubstrate, the surface of the steel substrate being partly exposed tothe air through penetrating cracks formed at the chromium layer.
 2. Theheat-treating method for improving the wear- and corrosion-resistance ofa chromium-plated steel substrate as set forth in claim 1, wherein thestep of heating in an oxidizing gas environment to form oxidized layersis carried out at 180˜570° C. for 10˜600 minutes.
 3. The heat-treatingmethod for improving the wear- and corrosion-resistance of achromium-plated steel substrate as set forth in claim 1 or 2, whereinthe oxidized layers formed by heating in an oxidizing gas environmentcontain magnetite (Fe₃O₄) in an amount of above 60% by weight.
 4. Theheat-treating method for improving the wear- and corrosion-resistance ofa chromium-plated steel substrate as set forth in claim 1 or 2, whereinthe oxidizing gas is steam, carbon dioxide, air, or mixtures thereof. 5.The heat-treating method for improving the wear- andcorrosion-resistance of a chromium-plated steel substrate as set forthin claim 1 or 2, further comprising the step of carbonitriding thechromium-plated steel substrate prior to heating in an oxidizing gasenvironment to form oxidized layers, thereby forming iron nitrides(Fe_(2˜3)N) in the form of a ε-phase on the surface of the steelsubstrate exposed to the air through fine cracks formed at the chromiumlayer.